In the research for 1974-75, I shall continue to use the readily accomplished procedure for extracting from acetone-dried human pituitaries (National Pituitary Agency) fractions of Somatotropin (h- STH) and Mammotropin (h-MH). All of the 4 main fractions processed through HCl-acetone and NH3-acetone and iso-electric precipitation have approximately 1 i.u. (mg of STH, but vary as much as 8-fold in MH). These hormones and others are assayed simultaneously in 1 month old hypophysectmized female rats. Having learned that the 2 fractions with the highest MH activity contain approximately 0.1 percent ACTH activity I shall attempt to segregate the latter. Although the contamination is almost negligible, it might conceivably interfere with a precise tibial cartilage growth assay. My other 2 fractions with equivalent STH but less MH activity have been found to contain approximately 1 percent LH. These are re-extracts of the 2 major fractions and have had less exposure than usual to HCl solutions at pH 1.5. They are also precipitated at 80 percent acetone; whereas STH and MH are soluble. Since I have shown that my best MH and some STH fractions are luteotropic in rats (without detected LH) I shall nevertheless attempt to segregate the LH and check it for LTH activity.